1. Field of the Invention
This invention relates to the dissolution of gases in liquids. More particularly, it relates to the oxygenation of large bodies of water.
2. Description of the Prior Art
Liquid waste destruction is commonly achieved at low cost by slurry-phase biotreatment processes in lagoons, surface impoundments and large tanks. In such processes, biological organisms, which may be either indigenous to the waste body or seeded therein from an external source, consume toxic, organic contaminants present in the waste body and convert them to less harmful substances.
For such biotreatment purposes, aerobic organisms are most commonly employed because, in general, they destroy organic contaminants much faster than anaerobic organisms. It will be appreciated that oxygen must be supplied to such processes in order to maintain a high contaminant destruction rate.
Surface aeration is a common oxygen supply method that can be used in slurry phase biotreatment operations. Such surface aeration is disclosed in the Haegeman patent, U.S. Pat. No. 4,468,358. In this approach, water is pumped from a waste body into the air for the entrainment and dissolution of oxygen therein. An effective oxygen transfer efficiency of approximately 1.9-2.6 lb/hp-hr can be achieved thereby. Surface aeration methods can cause severe foaming and, because they promote intimate contact between the waste material and the surrounding air, result in very high, undesirable organic chemical air emissions.
Air sparging is another common method for supplying oxygen to waste bodies for such biotreatment purposes. However, conventional air spargers typically result in the dissolution of only 5-10% of the oxygen injected into waste bodies thereby. Thus, for example, approximately 50-100 scfm of air must be injected into the waste bodies in order to dissolve 1 scfm oxygen. In addition, air sparging can cause unacceptable levels of organic chemical emissions as a result of the stripping action of waste oxygen and nitrogen on volatile compounds, when present in the waste bodies being treated. Severe foaming can also occur during air sparing operations.
If air is replaced by pure oxygen for biotreatment purposes, a much smaller feed gas volume is required to achieve the same dissolved oxygen level achieved by air sparging, and greatly reduced air emission levels result. However, most of the injected pure oxygen must be dissolved for such processing to be economical. In addition, the composition of any off gas must be outside the flammability limits of organic chemicals contained in the lagoon or other body of waste liquid.
Slurry phase biotreatment has been practiced, in a so-called MIXFLO.TM. approach, by pumping a side stream slurry from a tank or lagoon and injecting pure oxygen therein. The resulting two phase mixture is then passed through a pipeline contactor where approximately 60% of the injected oxygen dissolves. The thus-oxygenated slurry and the remaining undissolved oxygen are then re-injected into the tank or lagoon by passage through liquid/liquid eductors. About 75% of the undissolved oxygen remaining at the eductor inlet is thereby dissolved, resulting in the overall dissolution of 90% of the injected oxygen. The pumping power required for this application is relatively high, i.e., having an effective oxygen transfer efficiency of about 2 lb/hp-hr.
The UNOX.RTM. Process is a surface aeration process using a pure oxygen-containing headspace. An effective oxygen transfer efficiency of 6.5-7.2 lb/hp-hr can be achieved using this process and system. This approach can cause severe foaming, and waste liquid must be pumped from a large tank or lagoon to an external tank reactor, treated therein, and returned to said large tank or lagoon. It is thus subject to appreciable pumping costs.
Two other approaches that likewise are carried out in covered, confined tank systems, are the Advanced Gas Reactor (AGR) and Liquid Organic Reactor (LOR) processes and systems of Praxair, Inc. The AGR process and system, covered by the Litz patent, U.S. Re. 32,562, uses a helical screw impeller/draft tube assembly in a reactor to enhance the dissolution of oxygen from an overhead gas space. As the impeller turns, slurry is pumped through the draft tube so as to create, together with baffles positioned at the top of the draft tube, vortices in the pumped liquid, resulting in the entrainment of gas from the reactor headspace. Any gas not dissolved in a single pass through the draft tube is recirculated to the headspace and recycled. The AGR approach has an effective transfer efficiency of approximately 10 lb/hp-hr (standard transfer efficiency of 17-18 lb/hp-hr), and results in the dissolution of nearly 100% of the oxygen introduced into the system. It also ingests and destroys foam upon its passage through the draft tube.
The LOR process and system, covered by the Litz et al. patent, U.S. Pat. No. 4,900,480, is designed to safely dissolve oxygen in organic chemical-containing liquids. In certain embodiments, a horizontal baffle is positioned above the impeller/draft tube so as to provide a quiescent zone of liquid above the zone intended for gas-liquid mixing. Oxygen is injected directly into the impeller zone at a rate sufficient to sustain a high reaction rate, but low enough to maintain the oxygen level below the flammability limits of organic reactor contents. The LOR approach, like the AGR, consumes less power per pound of oxygen dissolved than pumping systems, the effective transfer efficiency of the LOR being approximately 10 lb/hp-hr.
Both the AGR and LOR approaches are carried out in covered, confined tank systems. Because of the tank requirements thereof and because of the additional foaming problems associated with the UNOX approach referred to above, further improvements in oxygen dissolution are desired in the art. Such improvements, in particular, are desired in light of the high power requirements associated with MIXFLO.
It is an object of the invention, therefore, to provide an improved approach to the dissolution of oxygen in liquids.
It is another object of the invention to provide a system for the efficient dissolution of oxygen in large liquid bodies.
With these and other objects in mind, the invention is hereinafter described in detail, the novel features thereof being particularly pointed out in the appended claims.